Stability, ranks, and the PhyloCode

Current codes of biological nomenclature define taxon names using types and ranks: the type determines the minimal membership of a named taxon, and the rank is supposed to determine its limits. Homo is “the taxon including the type species Homo sapiens that is assigned to the rank of genus”. However, there is no “genus concept” (analogous to a species concept), and thus no way of empirically determining the limits of a particular genus, even in the context of a single agreed phylogeny. The same problems also apply to higher taxa at all other ranks under current codes, leading to great taxonomic instability. All proposed objective criteria for determining membership of taxa at a particular rank (e.g., geological age, genetic divergence) are shown to be problematic. In contrast, the clades named by phylogenetic definitions are objective and stable. Node−based and branchbased definitions are most precise; however, apomorphybased definitions can be ambiguous due to difficulty in defining alternative character states, and optimisation uncertainty. A major benefit of ranks (information about relative nesting of taxa) can be achieved even more efficiently using standardised but rankless suffixes already widely used in phylogenetic taxonomy. Finally, in situations where the phylogeny is poorly known, phylogenetic nomenclature appears to be superior to the Linnean system. Phylogenetic nomenclature does not force one to officially name poorly corroborated groupings, whereas Linnean codes compel users to erect and name genera even when relevant supraspecific relationships are poorly known